A positive temperature coefficient thermistor is a heat generator having a self temperature control function, which is used in various devices as a constant temperature heat generator requiring no temperature control circuit.
Further, such a positive temperature coefficient thermistor is also a safe heater having a function of preventing superheating, which is hardly influenced by the ambient temperature.
FIG. 4 shows an example of a positive temperature coefficient thermistor having the aforementioned characteristics, which is used in a heater. Employed herein is a positive temperature coefficient thermistor 10, which comprises a positive temperature coefficient thermistor element assembly 11 and electrodes 12 formed on both major surfaces thereof. One of the electrodes 12 is connected to an object 13 to be heated through an insulating sheet 16, so that the positive temperature coefficient thermistor 10 is fixed to the object 13.
When the temperature of the aforementioned object 13 is sufficiently low, i.e., when the temperature of the positive temperature coefficient thermistor 10 is sufficiently low, it is possible to supply high output to the positive temperature coefficient thermistor 10 due to a low resistance value of this positive temperature coefficient thermistor 10. In order to heat the object 13 to a desired target temperature (target temperature), therefore, it is possible to quickly raise the temperature of the object 13 by supplying high output up to an intermediate stage of heating, e.g., to about 50% of a temperature-rise width for the target temperature, as shown in FIG. 5. However, beyond the stage of about 50% of the temperature-rise width, for example, the temperature of the positive temperature coefficient thermistor 10 rises at a higher speed than that of the object 13 to reach a level in a stable temperature region (temperature control region), and hence a amount of the supplied power is extremely reduced. As a result, the speed for raising the temperature of the object 13 is so disadvantageously reduced that a long time is required for raising the temperature of the object 13 to the target level.
On the other hand, there is used a heat generator having such a structure that a bimetal member 14 serving as temperature control means is thermally coupled to an object 13 to be heated and a high output heating element 15 of a foil heater or the like, for example, is connected in series to the bimetal member 14, as shown in FIG. 6. In this heat generator, it is possible to supply the high output heating element 15 with power which is greater than that required for maintaining the object 13 at a target temperature, due to a low resistance value of the high output heating element 15. Therefore, the temperature of the object 13 is raised at a high speed, while dispersion, i.e., pulsation disadvantageously appears in the temperature of the object as shown in FIG. 7, since the bimetal member 14 repeats ON and OFF states to carry out temperature control.
In addition to this, the heat generator shown in FIG. 6 has a mechanical contact due to the employment of the bimetal member 14, while switching between the ON and OFF states is repeatedly carried out. Therefore, noises are caused by such repetition of the ON and OFF states, while a malfunction is easily caused by deterioration of the contact.